1. Field of the Invention
The present invention relates to a cam mechanism for moving, for example, a lens group, such as a focusing lens or a zoom lens, in a photographing lens system.
2. Description of the Related Art
In a zoom lens camera, a cam mechanism having a cam ring which is provided in a lens barrel block of a photographing lens so as to rotate or linearly move in the optical axis direction thereof and a cam follower which is fitted in a cam groove formed in the cam ring and which is secured to a lens holder of a zoom lens group is provided to move the lens group (a variator and a compensator) in accordance with the photographing mode (zooming).
The cam ring is usually provided on its inner peripheral surface with a spiral cam groove. In case of a cam ring made of plastic or the like, having a cam groove integrally formed therewith, the cam groove is shaped as an outwardly widened truncated cone (generally V or trapezoid shape) to provide a draft so as not to produce an undercut portion in the cam groove. The cam follower has a truncated conical front end corresponding to the sectional shape of the cam groove.
In the cam mechanism including the cam groove and the cam follower, the cam follower can not be immovably secured to the lens holder itself in the direction of the cam groove. In practice, to fit the cam follower in the cam groove without play, the cam follower is supported on the lens holder so as to move in the direction of the cam groove, taking into consideration a possible change in shape of the cam follower due to change in temperature and the friction between the cam follower and the cam groove. Furthermore, the cam follower is biased to move into the cam groove by a spring, or the like.
However, in the mechanism mentioned above, there is a possibility that the cam follower largely projects from or comes out of the cam groove due to the spring force of the biasing spring. Accordingly, upon assembly, the lens holder must be carefully attached to the cam ring to prevent the cam follower from coming out of the cam groove, which is troublesome.
To solve this problem, it is also known to integrally form the cam follower with the lens holder and provide a spring force to the portion of the lens holder in which the cam follower is provided.
However, in this solution, it is very difficult to realize an appropriate spring force, and if the spring force is too strong, the cam ring can be easily deformed or broken. Furthermore, even if an appropriate spring force is realized, it is difficult to place the cam follower in the cam groove at an appropriate entrance position.